The present invention relates to a liquid crystal display device, more specifically, a liquid crystal display device which allows a defective pixel to be repaired to be normal, a method for repairing the liquid crystal display device, and a method for driving the display device.
A liquid crystal display device makes image displays by applying electric fields to a liquid crystal layer sandwiched between glass substrates to change an alignment direction of the liquid crystal to thereby change optical property of the liquid crystal. Recently, TFT liquid crystal display devices, which control the voltage to be applied to the pixel electrodes arranged in a matrix by thin film transistors (TFTs), are dominant.
The TFT liquid crystal display device often has point defects due to operational defects of the thin film transistors which are the driving element. Causes for the operational defects of the thin film transistors are various, e.g., formation defects of the channel unit, the inter-layer short, the intra-layer short between the source and the drain, etc. and are problems which have not yet been solved.
The TFT liquid crystal display device has a large number of pixels arranged in a matrix, and it is very difficult to perfectly prevent the generation of such defects. Allowing defects of a part of the pixels to make the device as a whole effective lowers the product yield and resultantly increases the fabrication costs.
Various methods for repairing such point defects of the liquid crystal display have been proposed.
A first method is for making a point defect not a bright point but a dark point to thereby make the point defect inconspicuous.
The display mode of the liquid crystal is largely classified in normally white (NW) mode, in which the white color display is made with no voltage applied and normally black (NB) mode, in which black color display is made with no voltage applied. The typical mode of the NW mode is TN mode, and the typical modes of the NB mode are MVA and IPS.
In the NW mode, the black color state is made possible with a voltage being applied to the liquid crystal. This state can be realized by connecting, e.g., the source electrode to the gate bus line by laser repair. A minus potential for tuning off the TFTs is almost always applied to the gate bus line, which allows the liquid crystal to turn on.
Reversely, in the NB mode, the black color state is always made possible with no voltage being applied. This state can be realized by connecting, e.g., the pixel electrode and the auxiliary capacitor bus line by laser repair. The auxiliary capacitor bus line must have a stationary potential so as to stabilize the pixel potential and generally is connected to the common electrode to thereby have the same potential. Accordingly, the pixel electrode and the auxiliary capacitor bus line are interconnected, whereby the above-state can be realized.
The first method described above is disclosed in, e.g., Reference 1 (Japanese published unexamined patent application No. Hei 05-027262), Reference 2 (Japanese published unexamined patent application No. Hei 07-020829), Reference 3 (Japanese published unexamined patent application No. 2001-305500), etc.
In a second method, the luminance of a pixel having the point defect is interlocked with a luminance of pixels around said pixel to thereby make the point defect inconspicuous.
Specifically, patterns have parts overlapping each other between the drain bus line and the pixel electrode, and when a defect takes place, the drain bus line and the pixel electrode are interconnected by laser repair. The voltage of the drain bus line is always applied to the pixel electrode, whereby the point defect is substantially invisible when all the screen has the same gradation (all black color, all white color, all gray color or others) and considerably inconspicuous in a picture display.
Methods in which the picture electrode is connected to an adjacent picture electrode in place of the bus line are also proposed.
The second method described above is disclosed in, e.g., Reference 4 (Japanese published unexamined patent application No. Hei 09-127549), Reference 5 (Japanese published unexamined patent application No. Hei 11-282007), Reference 6 (Japanese published unexamined patent application No. Hei 11-305260), etc.
In a third method, a plurality of thin film transistors are provided, and when a point defect takes place, the thin film transistor is disconnected to thereby repair the point to be normal. For example, 2 thin film transistors are provided for the usual use, and when a point defect takes place, the defective thin film transistor alone is disconnected. Otherwise, 2 thin film transistors are provided, and one of them is actually connected to a pixel, and when a point defect takes place, the pixel is disconnected from the thin film transistor which has been used to be connected to the spare thin film transistor.
The third method described above is disclosed in, e.g., Reference 1, Reference 7 (Japanese published unexamined patent application No. Hei 06-003694), etc.
The other related arts are disclosed in, e.g., Reference 8 (Japanese published unexamined patent application No. 2001-056652), etc.
However, in the above-described first method for repairing the liquid crystal display device, although a dark point is less conspicuous than a luminous point, it cannot be said that such state is normal. Although a dark point is less conspicuous than a luminous point in black color display, actually, a dark point in white color display is conspicuous in green pixels, etc. and is a considerably conspicuous defect also in RGB monochromatic display, etc.
In the above-described second method for repairing the liquid crystal display device, in the display in which characters, etc. are displayed side by side, the display in which the screen is bisected vertically in white color and black color, a point defect can be a dark point or a luminous point, and the point defect is rather conspicuous.
Furthermore, when pixel electrodes adjacent to each other along the extension of the gate bus line are interconnected, these pixels are for displaying different colors. When an image mostly in one color is displayed, the pixel can be recognized as a point defect. When pixel electrodes adjacent to each other along the extension of the drain bus line are interconnected, the pixels are for the same color, which makes the point defect inconspicuous. However, in images, as of stripe patterns of every but one line, zigzag patterns, etc., the pixel appears as a point defect.
In either of the cases, in which adjacent pixel electrodes are interconnected, 2 pixels are driven by one thin film transistor. In terms of the writing capacitor and the parasitic capacitor, the displays of the pixels including the adjacent pixel tend to be abnormal. There is a risk that the defect of one pixel may be extended to the defect of the two interconnected pixels and worsened.
In the above-described third method for repairing the liquid crystal display, after paneled, the repair is made while watching at the underside, which has made it very difficult to judge which thin film transistor is defective. Which thin film transistor to be disconnected has had recourse to knack, and the success rate has been low. Furthermore, a normal point has 2 thin film transistors, but an abnormal has 1 thin film transistor. Depending on a display state, the defect is often visible.
In many cases, causes for the point defect take place in a small region. Even when 2 thin film transistors are provided for 1 pixel, it is very possible that the 2 thin film transistors may become simultaneously defective. As a countermeasure to this, it is an idea to space the 2 thin film transistors from each other. In such case, the area except that area of the display unit is increased, and the aperture ratio is much decreased.